Zeolite membranes have long been a goal of separation science. For a zeolite membrane to be practical, it must have a high flux as well as selectivity. Obtaining such a membrane has been difficult in the past because of defects in the zeolite film. This has been especially true for membranes grown from low alkaline synthesis routes described in the literature. These membranes have a heterogeneous crystal structure in the membrane and require an enormous (&gt;50 micron) layer thickness to seal pinholes and void structures. What is needed in the art is a thin zeolite membrane with very few defects.
A patent describing the direct synthesis of zeolite membranes has been issued to W. Haag and J. G. Tsikoyiannis of Mobil (U.S. Pat. No. 5,110,478, issued May 5, 1992). A paper describing scientific results obtained with this type of membrane was published in an article titled "Synthesis and Characterization of a Pure Zeolitic Membrane" by J. G. Tsikoyiannis and W. Haag in Zeolites (Vol. 12, p. 126, 1992). The membrane described in the above article and patent is used as a freestanding membrane and is not affixed or attached as a layer to a microporous support making it mechanically fragile and leading to ready breakage during use. The physical structure of the membrane is such that there is a gradient of crystal sizes across the thickness of the membrane. This gradient of crystal sizes throughout the layer thickness precludes growth of a thin membrane with a minimum number of non-selective permeation paths.
Zeolites have also been grown on supports. See, for example, "High temperature stainless steel supported zeolite (MFI) membranes: preparation, module construction and permeation experiments," E. R. Geus, H. van Bekkum, W. Bakker and J. Moulijin, Microporous Materials, Vol. 1, p. 137, 1993; Netherlands patent application 9011048; European patent application 91309239.1 and U.S. Pat. No. 4,099,692.
All of the above prepared membranes are formed with several zones (larger crystals grown on top of smaller crystals) across the membrane thickness. In several zones, the crystals are not grown into a dense mat that is free of intercrystalline voids. To obtain a permselective zeolite membrane, the above zeolite layers (comprised of zones) must be grown to an excessive thickness (&gt;50 microns) to seal off voids and defects within the membrane. This creates a great mass transfer resistance causing reduced flux. Obtaining functional zeolite membranes from high alkaline synthesis routes is difficult because the heterogenous crystals in the membrane require an enormous membrane thickness to seal pinholes and void structures which lowers the membrane selectivity. The presence of such pinholes and voids is the cause of optical scattering in as synthesized high alkaline membranes.